The battery portion of the update covers two areas. Firstly it will fix an issue that has resulted in some devices being unable to take advantage of the full capacity of the integrated battery. Secondly it improves the overall power efficiency of the device (e.g. power used in data connections). The overall improvement in battery life will vary from device to device and is also dependent on device usage patterns, but a typical user will see an improvement of around 10% to 15%.
The update will upgrade the firmware version number of the Nokia Lumia 800 to 1600.2483.8106.11500 (you can see number by tapping on the more information button in the about screen of the settings application).
Nokia Conversations notes that, as usual, the update will not be delivered to all devices simultaneously:
The update will not be delivered to every Lumia 800 simultaneously, but rather will be staggered over the next two weeks through Microsoft’s standard update process. This helps each owner gets the update without any hitches. Around half of owners will get the update in the next seven days, with the remainder over the following seven days.
Your Nokia Lumia smartphone will automatically check for updates from time to time, and you’ll get a message on the phone when it is ready.
This second device software update for the Lumia 800 follows on from the first update, which fixed a charging related power issue. Together they fulfill the commitment made by Nokia late last year to solve the battery issues that some user faces.
When its first two Windows Phone devices were announced in October 2011 Nokia indicated that Internet Sharing functionality would be added via a device software update. It is likely that this will be the main feature of the next device software update.
Note that this device update is separate to the minor platform update that is also currently being delivered to Lumia devices. In some cases the updates will be delivered together, with one applied immediately after the other.
More details on how the software update process works is available on the Nokia support pages.